The Sulak (Russian: Сула́к, Kumyk: Сулак (Sulak)/Къой-сув (Qoysuw), Chechen: Ġoysu[1]) drains most of the mountainous interior of Dagestan northeast into the Caspian Sea.

To the west is the crest of the Main Caucasian Range of the Caucasus Mountains and to the east are many short rivers that flow down into the Caspian.

The Sulak proper starts at the junction of the Andi and Avar Koysus.

It reaches flat country at Miatli and leaves the last mountains just south of Kizilyurt and enters the Terek-Sulak Lowland about 50 km (31 mi) from its source.

In 1735 the Sulak River was stipulated as defining the boundary between the Russian and the Persian empires.